The present invention relates to a method of and apparatus for grinding material particles such as cement clinker, slag or ore, and a grinding apparatus provided with a classifying device for performing classification of the material particles.
In a prior art technique, when it is required to manufacture cement powders by grinding burned cement clinkers, a tube mill has been utilized as a grinding means. The tube mill comprises a tubular (cylindrical) rotary container in which a number of steel balls are accommodated, and the material particles to be ground are subjected to the grinding operation in the container by rotating the same.
In such a grinding operation, when the materials including coarse particles, each having a relatively large diameter, are to be ground, it is necessary to utilize balls each having a large diameter. However, when the balls having large diameter are utilized, contacting areas between the balls and the material particles are reduced, thus reducing the grinding efficiency. Accordingly, in recent years, a roller mill is often used for precedingly grinding the material particles including coarse particles in combination with the tube mill.
The roller mill, for example, a vertical roller mill, has a structure in which peripheral surfaces of a plurality of rollers are pressed against an upper surface of a table rotating in a horizontal plane and the material particles supplied on the table are ground by being pressed by the rollers. As described, since the coarse materials can be effectively ground by applying sufficient pressing force to the rollers, the roller mill is effective for the coarse grinding operation for the succeeding operation for the tube mill. When the material particles are thus ground to fine particles by means of a roller mill, the grinding efficiency of the tube mill can be improved by using a smaller size of steel balls.
FIGS. 3 and 4 shows examples of conventional grinding apparatus constructed as a combination of a roller mill and a tube mill.
FIG. 3 shows an arrangement of the grinding apparatus disclosed in the Japanese Patent Laid-Open Publication (KOKAI) No. SHO 63-116751 (116751/1988), in which a vibration screening device 1 having screens 1a as a classifier is arranged in a material conveying path 20 extending from a vertical roller mill 10 having a roller 12 therein to a tube mill 40. A fine powder component classified by the vibration screening device 1 is fed to the tube mill 40 through the material conveying path 20. A coarse particle component classified thereby is returned to the roller mill 10 through a material circulation path 25.
FIG. 4 shows an arrangement of the grinding apparatus disclosed in the Japanese Patent Laid-Open Publication (KOKAI) No. HEI 4-338244 (338244/1992), in which a distributing device 2 is disposed in place of the vibration screening device 1 of the example of FIG. 3. The distributing device 2 is provided with a butterfly-damper-type plate member 2b variable in angle in a fork-shaped case 2a. An amount of distribution of particle material can be adjusted in directions to be supplied by adjusting the angle arrangement of the plate member 2b. That is, as shown in FIG. 4, one forked portion 2a is connected to the material conveying path 20 connected to the tube mill 40 and the other forked portion 2a is connected to the circulation path 25 connected to the vertical roller mill 10 having roller 12 installed therein. Thus, in this example, a portion of the particle material subjected to a pre-grinding operation is fed to the tube mill 40 and a remaining portion thereof is returned to the roller mill 10.
In both examples of FIGS. 3 and 4, the material ground by the tube mill 40 is then fed to a separator 52 through a bucket elevator conveyor 51, and the material finally ground by the tube mill 40 is separated by the separator 52 to a material component capable of being utilized as a product and another material component to be re-ground.
In the example of FIG. 3, the fine material component, in the pre-ground material, classified by the vibration screening device 1, i.e. the material passing through meshes of the screening device having predetermined mesh size, can be fed to the tube mill 40. Accordingly, it is possible to improve the grinding efficiency in maximum by utilizing balls each having a small diameter. However, this example involves a problem in relation to the fact that only the coarse material exactly classified by the vibration screening device 1 is returned to the roller mill 10. Since the material returned to the roller mill 10 does not substantially include a fine material component, a high percentage of void is provided in the material particles fed to the roller 12, which may result in violent vibration or oscillation at the line of grinding of the roller mill 10.
In order to suppress or control such vibration, it is necessary to limit a pressing force of the rollers 12 disposed in the roller mill 10. This, however, provides a significant problem such that the roller mill 10 can not achieve sufficient grinding efficiency in a pre-grinding process. In addition to the above defect, in the example of FIG. 3, an extremely large-sized and expensive vibration screening device 1 is required, and it is necessary to arrange various screens 1a having different mesh sizes to change the classifying levels or degrees. Moreover, since an amount of the material to be fed to the tube mill 40 is decided in accordance with the screens 1a utilized, which is not controlled during the operation of the apparatus, it is difficult to maintain or control the amount of the product stably.
On the other hand, the example of FIG. 4 is an apparatus provided for obviating the problems of the apparatus of FIG. 3. That is, in this apparatus, as described above, the fine particles are also fed to the roller mill 10 together with the coarse particles, thus providing a relatively low percentage of void in the feeding materials to the roller, and accordingly, a reduced vibration is caused during the re-grinding operation in the roller mill 10, whereby the grinding efficiency in the roller mill 10 can be improved, thus being advantageous in comparison with the example of FIG. 3.
On the contrary, however, the apparatus of FIG. 4 is inferior to that of FIG. 3 in a point that the coarse material is also fed to the tube mill 40 together with the fine material subjected to the pre-grinding operation. Therefore, it is necessary for the tube mill to use balls each having a large diameter, which results in the lowering of the grinding efficiency in the tube mill as mentioned hereinbefore.
Accordingly, in both the examples of the conventional grinding apparatus of FIGS. 3 and 4, it is difficult to achieve the improved grinding efficiency both in the roller mill and the tube mill.